Add CVar to enable original Doom health mechanics

* Disables health token system when enabled
* Added logic to set health tokens as a function of player health
* Added CVar-aware spawners for Doom health and armour pickups
* Changed player damage resistance to 0.5 to compensate for OHGODTHEPAIN
* Improves balance in mods designed around the original health system

This and the dropammo branch will need conflicts fixed if they're both merged, see the previous pull request for more details.